The treatment of autoimmune and other inflammatory diseases such as RA and SLE has entered a new and exciting phase where increased understanding of the molecules involved in the immune system has allowed for the specific inhibition of key inflammatory molecules such as tumour necrosis factor-a (TNFa) and interleukin 1p (IL-1(3). For example, in recent studies, it has been shown that antibodies can play a powerful role in the pathogenesis of RA, and in human clinical trials, positive responses to the use of anti-CD20 monoclonal antibody (MAb) therapy to eliminate antibody producing B cells have been generating strong evidence of the significant role of antibodies in RA (Emery et al., 2001). Since Fc receptors (FcR) play pivotal roles in immunoglobulin-based effector systems, inhibition of FcR function may provide the basis of effective therapy for a variety of diseases. Moreover, since Fcy receptors (FcyR) are pivotal to effector systems for IgG, targeting the interaction between leukocyte FcyRs and antibodies provides a new opportunity for therapeutic intervention in RA (Nabbe et al., 2003). One approach of achieving such an intervention which is of interest to the present applicants is the use of a soluble form of an FcγR to act as a “decoy” to prevent leukocyte activation by antibodies.
Fc receptors (FcR) are leukocyte surface glycoproteins that specifically bind the Fc portion of antibodies. The receptors for IgG, that is FcγR, are the most widespread and diverse, the major types being FcγRI (CD64), FcγRII (CD32) and FcγRIII (CD16). Immune complexes (IC) that are formed in vivo in normal immune responses, and those seen in the pathology of autoimmune diseases such as RA, can simultaneously engage many FcR. For example, in humans, activated macrophages, neutrophils, eosinophils and mast cells can express FcγRI, FcγRIIa, FcγRIIb and FcγRIII (Takai, 2002). However, of these, the FcγRIIa is the major initiator of IC-mediated inflammation and, while all of the FcγR types engage the lower hinge region of the IgG Fc domain and the CH2 domains such that any soluble FcγR decoy polypeptide might inhibit the binding of IgG to all classes of FcγR, the present applicants have realised that since FcγRIIa shows the widest binding specificity and highest selectivity for avid IgG immune complex binding, the development and investigation of a soluble FcγRIIa offers the greatest potential.
Indeed, previous studies have shown that a simple recombinant soluble FcγRIIa polypeptide (rsFcγRIIa monomer), consisting of FcγRIIa ectodomains (Ierino et al., 1993a), is clearly able to inhibit IC-mediated inflammation. In these studies, the rsFcγRIIa was tested using the Arthus reaction, wherein immune complexes are formed in the dermis by the passive administration of antibody and antigen (Pflum et al., 1979), which is a model of vasculitis (an extra articular complication in arthritis) and also occurs in SLE. It was found that while the rsFcγRIIa monomer inhibited inflammation and neutrophil infiltration when co-administered with the antibody and antigen, large amounts of the rsFcγRIIa monomer were required because of a relatively low level of selectivity for the immune complexes. To overcome this problem, the present applicants proposed to use multimeric forms of the rsFcγRIIa decoy, and has since found, surprisingly, that not only could such multimeric forms be successfully expressed, they exhibit increased selectivity for immune complexes. Such multimeric rsFcγRIIa polypeptides therefore show considerable promise for the treatment of IC-mediated inflammatory disease such as RA and SLE.